Various commercial and military aircraft often make use of a cabin air (CA) inlet for the environmental control system (ECS) that such aircraft employ. Aircraft making use of a CA inlet employ ram air for cabin pressurization rather than bleed air from the engines. With such aircraft, the ram air captured by the CA inlet is often fed to an electric motor driven cabin air compressor (CAC), then conditioned to the desired temperature and pressure in an air conditioning pack, and then supplied to the air distribution system of the cabin.
An important requirement when using ram air to feed a cabin air compressor is achieving a minimum desired recovery factor (RF) at the CAC inlet face. The term “Recovery Factor” may be defined as: Recovery Factor=(Total pressure recovered by the inlet—Free-stream static pressure)/(Free-stream total pressure—Free-stream static pressure). The Boeing Company has also used this terminology in providing design requirements specifications to its suppliers. The same parameter has also been variously called “inlet efficiency”, “ram pressure efficiency”, “ram-recovery ratio”, etc. In all cases the definition of the parameter is the same. The parameter was originally defined by NACA (U.S. National Advisory Committee for Aeronautics, the predecessor of NASA). In practice, it is desirable to achieve the maximum RF possible at the CAC inlet face in order to minimize the electric power required to drive the compressor(s) of the air conditioning pack. This is particularly important at the peak power condition because the generator, motors and other electrical equipment of the ECS need to be sized to meet the peak demand requirements of the aircraft. Ideally, the RF achieved at the CAC inlet face would be 1.0, but in practice it is typically considerably less than 1.0, and often around 0.05-0.7. On the other hand, however, a higher RF for a ram air inlet is generally associated with a higher drag. Therefore, a design challenge is present in providing an inlet for an environmental control system component of the aircraft, and more particularly for a cabin air inlet, that is able to achieve a predetermined minimum RF, while also minimizing the drag of the inlet.
In the presence of a thick fuselage boundary layer, flush mounted ram air inlets (rectangular or NACA planform) that are positioned flush against the exterior surface of the fuselage of the aircraft, and which are of the type used for supplying cooling air to an air conditioning pack heat exchanger, tend to yield a RF in the range of about 0.6 to 0.7. However, due to limitations on available compressor power, it is desirable to achieve a RF closer to 1.0, and at least about 0.8, to make most efficient use of the air inlet. Therefore, present day, flush mounted ram air inlets often fall short of the ideal performance parameters. Furthermore, at low mass flows, flush mounted ram air inlets are also prone to develop an undesirable Helmholtz type duct flow instability, which arises from a coupling between acoustic resonance in the duct and separation of the approaching boundary layer ahead of the inlet. Thus, a concurrent performance consideration, in connection with maximizing the RF performance of the inlet, is to minimize the drag associated with the implementation of the inlet while simultaneously providing an inlet that is able to delay the onset of flow instability to significantly lower mass flows.
Still a further concern is the ability of locating a cabin air inlet relative to the location of one or more additional inlets that are typically used in connection with an environmental control system on an aircraft. For example, on commercial and military aircraft, one or more inlets are used to supply airflow to one or more cabin air compressors, while one or more heat exchanger ram air inlets are also incorporated for supplying cooling air to a heat exchanger of an air conditioning pack on the aircraft. It would be desirable if the heat exchanger inlet could be placed relative to the cabin air inlet in a manner that modifies the boundary layer immediately upstream of the cabin air compressor. This would allow the optimum performance characteristics of the cabin air inlet to be met while still reducing drag associated with the cabin air inlet.